It is customary to use pneumatically driven tools in the assembling of fasteners, such as nuts and bolts, to connect parts in an assembly line operation. Machines are designed to be built with each fastener receiving a specific amount of torque, which may differ from fastener to fastener. Insufficient torque results in loose screws. Too much torque strips the threads.
It is generally left to the discretion of the operator to apply the proper amount of torque to each fastener. A skilled operator can judge the proper amount of torque by the sound of the tool and by the length of time air is delivered to the tool. The necessary skill is lost with a turnover in personnel, resulting in defective parts.
Most manufacturers employ full time quality control inspectors to insure that the bolted assembly is within torque specifications. Many quality control inspectors use a click-type torque wrench to measure the torque. This provides a satisfactory test for insuring a minimum torque up to 100 foot pounds of pressure, but it cannot indicate excessive torque, which might overstress the bolt or strip the threads.
The repeated use of a click-type torque wrench to apply 100 foot pounds of pressure throughout the day is so tiring that a quality control inspector may make measurements toward the end of the day that vary from the true torque by as much as twenty five percent.
The additional labor expense of hiring full time quality control inspectors and the ineffectiveness of using click-type torque wrenches can be overcome by installing computer operated assembly machines with built-in torque control that is accurate within a tolerance of five percent. The principal objection to such machines is their cost, which renders them unavailable to all but the larger manufacturers.